1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus for applying the power of an electric motor as an assistive steering force to a mechanical steering system to reduce the manual force required to steer a motor vehicle which incorporates such an electrically operated power steering apparatus.
2. Description of the Prior Art
One known electrically operated power steering apparatus, which is disclosed in Japanese laid-open patent publication No. 62-238165, has a steering torque detecting means for detecting the steering torque of a steering system, a steering rotational speed detecting means for detecting the steering rotational speed of the steering system, a returned state detecting means for detecting a returned state of the steering system, a vehicle speed detecting means for detecting the speed of a motor vehicle which incorporates the electrically operated power steering apparatus, and a correcting means for limiting the maximum value of an output signal from the steering rotational speed detecting means to a value which decreases as the vehicle speed based on an output signal from the vehicle speed detecting means increases. A control signal for controlling an electrical motor which generates an assistive steering force is determined on the basis of an output signal from the steering torque detecting means and an output signal from the correcting means.
The conventional electrically operated power steering apparatus operates as follows: The maximum value (D.sub.NMAX) of a returning rotational speed control signal for the electric motor which decreases as the vehicle speed V increases, and rotational speed control signals (D.sub.N) for the electric motor which correspond to steering rotational speeds N are stored as a table in a memory. When a returned state of the steering system is detected by the returned state detecting means, the maximum value (D.sub.NMAX) of the returning rotational speed control signal is compared with the rotational speed control signal D.sub.N. Even when the rotational speed control signal D.sub.N exceeds the maximum value (D.sub.NMAX) of the returning rotational speed control signal (D.sub.N &gt;D.sub.NMAX), the rotational speed control signal D.sub.N is reduced to the maximum value (D.sub.NMAX) of the returning rotational speed control signal. When the motor vehicle runs at a low speed, the returning speed of the steering system is held at a relatively large level to allow the steering system to have an increased returning capability, and then the motor vehicle runs at a high speed, the returning speed of the steering system is lowered to increase steering system returning stability.
Therefore, when the motor vehicle runs at a low speed, the conventional electrically operated power steering apparatus maintains a high returning speed of the electric motor for increased returning performance, and when the motor vehicle runs at a high speed, the conventional electrically operated power steering apparatus reduces the returning speed of the electric motor as the vehicle speed increases, thereby shortening the time required for the steering system to converge to a neutral position, so that the returning stability can be improved.
The conventional electrically operated power steering apparatus utilizes a voltage induced by the electric motor to vary damping characteristics of the steering system for thereby improving steering characteristics of the motor vehicle. When the motor vehicle runs at a higher speed, since the motor vehicle behaves or responds more sharply than when it runs at a lower speed, the steering wheel tends to lose directional control due to road and tire conditions particularly at the time the motor vehicle moves across pavement joints or steps.
When the motor vehicle is steered in the vicinity of the resonant frequency of the yaw rate of the motor vehicle, the yaw rate gain of the motor vehicle is increased, causing the motor vehicle to behave more sharply and making the steering response insufficient as felt by the driver.
Consequently, the steering system is required to be given positive damping characteristics, and to be given greater damping characteristics.
The damping characteristics of the steering system may be improved by a mechanical damper device, such as a steering damper, connected to the steering system. However, inasmuch as the damping characteristics of the steering damper are determined by the viscosity of oil used, if the damping characteristics of the steering damper are established to maintain stability when the motor vehicle runs at a high speed, then the returning capability of the steering system may be lowered when the steering system has a low self-aligning torque such as at the time the motor vehicle runs on a road with a low coefficient of friction or at a low speed or the ambient temperature is low.
The mechanical damper device added to the steering system results in a large system size, making the system difficult to install and highly costly to manufacture.
Furthermore, since the damping characteristics for controlling the electric motor do not take into account any component relative to the steering torque, the steering system requires large manual steering forces as when the motor vehicle is to be turned quickly to avoid an object in emergency, for example.